1. Field of the Invention
The present invention relates to an electro-explosive device or initiator for an automobile passenger restraint electrical system, and more particularly, to an initiator which includes a metal oxide varistor for electro-static protection of the electro-explosive device.
2. Description of the Related Art
It is known in the prior art to employ an inflatable occupant restraint system for protecting a passenger of an automobile. Such restraint systems encompass a reaction canister which houses a gas generator or inflator, and an air bag in an uninflated condition. In response to a collision, the gas generator generates gas to inflate and expand the air bag to protect the vehicle occupant.
Inflators for automobile passive restraint systems or other devices require a pyrotechnic initiator or electro-explosive device (EED) to operate the inflator. For actuating the gas generator or inflator an electro-explosive device starts the material of the gas generator burning. The inflator initiator is connected to a crash sensor that is positioned adjacent the initiator or at a remote location in the vehicle.
In operation, the crash sensor sends an electrical signal to the initiator. The initiator fires into the ignitor chamber and ruptures a container, which holds an ignitor material, commonly a mixture of boron and potassium nitrate. The initiator consists of a pair of spaced parallel electrical pins joined at one end by a bridge wire which is embedded within pyrotechnic material. The pyrotechnic material bums with a very hot flame and ignites solid fuel gas generant pellets contained in the combustion chamber. The pellets release a nitrogen gas, which travels through the diffuser chamber and into the protective air bag for protecting occupants of the vehicle.
A common characteristic of electro-explosive devices (EED's) is the susceptibility of the bridge wire to undesirable energy from outside environmental sources which could lead to an inadvertent deployment or "dudding" of the initiator. The undesirable energy may be either, but not limited to, electro-static discharge (ESD), radiant electromagnetic interference (EMI) or radio frequency interference (RFI). Protection against such radiant energy will herein be referred to as EMI/RFI protection.
One prior art solution to overcome EMI/RFI hazard involves the use of a ferrite bead disposed directly within a chamber of the initiator. The ferrite bead absorbs the extraneous energy preventing the energy from reaching the bridge wire. See U.S. Pat. No. 4,306,499 to Holmes, which is assigned to the assignee of the present invention.
A problem with the electro-explosive device of Holmes is that incorporation of the EMI/RFI protection directly within the confines of the EED increases both the size of the device, as well as, manufacturing costs and time. Moreover, the manufacturer of the gas generator is limited to a specific EED design.
Another solution is a universal squib connector which encompasses a ferrite bead which surrounds the electrical terminal of the EED. See U.S. Pat. Nos. 5,200,574 and 5,241,910 to Cunningham et al., assigned to the assignee of the present invention. Cunningham et al. discloses a universal connector encompassing EMI/RFI protection, which are permanently secured within the gas generator. The ferrite bead, electrically, is essentially an inductor which impedes the instantaneous change in current flow.
Another problem with known inflator assemblies is that the initiator (EED) is crimped into the inflator base. This crimping process often damages the initiator if done improperly.
U.S. Pat. No. 4,103,619 discloses an electro-explosive device wherein the contact pins of the device are sealed with glass and a resistant shunt is provided for protecting the bridge circuit from extraneous energy. One disadvantage of the resistant shunt is a percentage of the energy being supplied to the initiator during the deployment sequence is shunted to ground, thus requiring substantially more total energy than would be needed if the resistant shunt was not present.
U.S. Pat. No. 4,422,381 discloses an ignitor incorporating a ferrite sleeve (EMI/RFI) and static discharge disc (ESD). The static discharge disc relies on a "spark gap" method for ESD protection and is limited in its usefulness to a specific type of electro-static discharge voltage, i.e. typically greater than 25,000 volts dc. The current invention utilizes the metal oxide varistor which can tailor the ESD protection to a wider voltage level range by varying the dimension of the varistor. The current invention is designed to shunt to ground any extraneous voltage in excess of 500 volts dc.
FIGS. 1-6 disclose various prior art initiators. FIG. 1 discloses a low cost ignitor, manufactured by ICI Explosives of Tamaqua, Pa., which incorporates a spark gap ring for ESD protection. FIG. 2 illustrates a hybrid initiator, also manufactured by ICI Explosives, which includes two pins and a shunt wire, in essence acting as a co-axial initiator. FIG. 3 illustrates another low cost initiator, such as the initiator manufactured by Quantic Industries of San Carlos, Calif. The prior art initiator of FIG. 4 is a pin-type initiator using an offset co-axial header design, such as the initiator manufactured by Special Devices Inc. of Newhall, Calif. Prior art FIG. 5 discloses a leadwire initiator having a centered co-axial design, also manufactured by ICI Explosives of Tamaqua, Pa. FIG. 6 is an advanced driver initiator having a co-axial centered design, manufactured by OEA Inc. of Englewood, Colo. One disadvantage of all of the prior art initiators of FIGS. 1-6, is the necessity of complete electrical isolation.
The use of a metal oxide varistor (MOV) to absorb electrostatic energy in an electro-explosive device has recently been explored. See, V. Menichelli, A Varistor Technique to Reduce the Hazards of Electrostatics to Electroexplosive Devices, (1974). See also U.S. Pat. Nos. 4,103,274, 4,041,436 and 3,821,686.
Typically, metal oxide varistors are used in surge suppression devices, such as computers. However, the prior art has not explored the use of a MOV in an electro-explosive device for use in an air bag gas generator.